The ever increasing complexity of today's electronic devices has led to more complex circuits being provided in smaller packages. Faster system clock speeds increase the chances of spurious and interfering emissions signals being created as well as raise issues with respect to impedance matching between device pins and connections. Signal lines are, at times, terminated with appropriate devices such as resistors or capacitors in order to match impedance and other passive components are provided in order to reduce transients and spurious signal transmissions. It is often desirable to locate the terminating or impedance matching device as close as possible to the signal source or load.
Surface mount (SMT) passive components, such as resistors, capacitors and inductors, are commonly provided on printed circuit boards (PCBs) to address these issues. Referring to FIG. 1, there is illustrated one example of a standard SMT resistor 1100. The resistor 1100 has a generally rectangular package body 1102 with two solderable pads 1104 for connecting to a printed circuit board. Surface mount passive components are generally fairly small—with a longitudinal length, L, of about 0.040 inches (±0.002 inches), a height, h, of about 0.018 inches (±0.002 inches), and a width, w, of about 0.020 inches (±0.002 inches). Despite the already small size of these components, the drive toward ever smaller electronic devices, e.g., cell phones and cameras, and thus, smaller and more densely populated printed circuit boards, makes placement of even passive SMT components on printed circuit boards challenging.
The Ball Grid Array (BGA) device presents issues when a terminating device, e.g., a resistor, needs to be connected and placed nearby. Referring now to FIG. 2, a BGA device 1110 has an underside surface 1112 with an array of solder balls 1114. The solder balls 1114 serve as contacts to an integrated circuit (IC) within the package. The array of solder balls 1114 is configured to attach, either mechanically by compression or when soldered, to a printed circuit board 1106 with a corresponding array of pads 1108 as conductive contacts on a surface 1109.
Referring to FIG. 3, there is illustrated an example of a BGA device 1110 with solder balls 1114 coupled to respective pads 1108 on the PCB 1106. As is known, the solder balls 1114 of the BGA device 1110 may be coupled to the pads 1108 of the PCB 1106 by soldering or by compression fit with the implementation of any one of a number of known mechanisms. The coupling is both mechanical and electrical in that an electrical signal from the IC within the BGA device 1110 will pass through a solder ball 1114 to its respective pad 1108 on the PCB 1106 and on to whatever other device or devices is connected to that pad 1108.
Generally, there is a very limited space or area in which to place a passive component, such as a resistor, whereby it is both on the PCB 1106 and physically close to a solder ball 1114 of a BGA device 1110, or its corresponding pad 1108. This is especially challenging for a solder ball 1114 or pad 1108 not at the outer portions of the array. This limited space creates a challenge for optimal component placement. As is known, with multi-layer printed circuit boards, vias are used to connect surface mount components to signal traces on other board layers as well as to the solder balls 1114 of the BGA device 1110. For example, referring back to FIG. 2, there is illustrated an example of a printed circuit board 1106 with pads 1108, vias 2112a and 2112b, and traces 2110a and 2110b, used to connect the surface mount resistor 1100 to the BGA device 1110. The trace 2110b, however, forms an extra stub length that can add undesirable inductance, thus possibly affecting performance or causing spurious noise, but which is required to connect the surface mount resistor 1100 to the BGA device 1110.
As shown in FIG. 4, a vertical stub is caused by a via 2112 connecting a surface mount resistor 1100 to inner signal layers of a multi-layer board. The surface mount resistor 1100 is mounted on either (the top or bottom, i.e., an outer surface, of a multi-layer printed circuit board and connected (by its solderable terminals 1104) to a signal trace 2110. The via 2112 is used to connect the surface mount resistor 1100 to signal traces 2116 on other layers. In the illustrated example, a via pad 2114 couples the via 2112 to a signal trace 2116a on layer two, thereby forming a series connection of the trace 2110, resistor 1100, and trace 2116a. The remaining length of the via 2112 from layer three to the bottom via pad 2118 on layer six is an extra vertical stub length that adds additional inductance and could act as an antenna.
Buried passive components are an alternative to surface mount passive components. A buried passive component, for example, a resistor, is screened onto an inner layer of the printed circuit board and is connected to traces and vias as needed. The buried component may be placed on an inner layer, possibly eliminating a via that would otherwise be needed to connect the component from the top or bottom of the board to a signal trace on an inner layer. Vias are still needed, however, to form connections between the signal traces on different layers. Thus, the presence of the buried component on the signal layer still requires valuable via and signal trace space on the inner layers of the board.
What is needed is a mechanism for providing passive components on a PCB that uses BGA devices as close as possible to a BGA contact without having to give up a large amount of PCB surface area to vias and signal traces.